1. Field of the Invention
This invention relates to polyurethane one-component baking enamels containing solvents and consisting of 2-methyl-1,5-diisocyanatopentane (MPDI) blocked totally or partially with .epsilon.-caprolactam or acetoxime or a mixture of MPDI and 2-ethyl-1,4-diisocyanatobutane (EBDI) and also of polyesters containing hydroxyl groups with a base of predominantly aromatic dicarboxylic acids and diols and triols.
2. Description of the Prior Art
For the production of mixtures of polyisocyanates and polyoxy compounds that are stable at the usual temperatures, as is known, polyisocyanates are used whose reactive groups are blocked by reaction with monofunctional compounds and generally are designated as masked or blocked polyisocyanates. Products of this type are described, for example, in Annalen, Volume 562 (1949) pp 205-229.
At higher temperatures, they again break down into their initial components, whereby the isocyanate groups, which are released, in turn can react with the polyoxy compounds that are present in the reaction mixture.
If these mixtures of blocked polyisocyanates and polyoxy compounds are to be applied for coating objects by the spraying or curtain coating process and for coating of sheet metal by roll coating, especially by the coil coating process, both the cross-linking agent and the resin must be soluble in the usual varnish solvents and stable in storage.
With only few exceptions, aliphatic polyisocyanates blocked with .epsilon.-caprolactam or acetoxime, are incompatible with the usual varnish solvents. They are, therefore, eliminated as initial components for production of PUR one-component baking enamels preferably for coating metal objects with highly deformable coatings.
Production of hexamethylenediisocyanate (HDI) blocked with .epsilon.-caprolactam or acetoxime in conventional varnish solvents is thwarted by incompatibility. These products precipitate during or shortly after the reaction. Even partial blocking of HDI and subsequent reaction with the polyoxy compounds was able to impart to the varnish solutions only a short-lived stability in storage. Another drawback is the limited thermal and oxidation stability, which in practice proves to be markedly troublesome when the conveyor is stopped during hardening; the results are quality losses, e.g., discoloration of the varnish layer.
Therefore, a need clearly continues to exist for aliphatic polyisocyanates which can be blocked with .epsilon.-caprolactam or acetoxime to form blocked polyisocyanates which are compatible with conventional varnish solvents and which exhibit in the varnish solutions both thermal and oxidative stability.